<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="../css/reset.css">
    <style>
        html {
            /* font-size: 1px; */
            /* 1rem = 1px  */

            /* 动态设置根元素的字体大小 */
            /* font-size: calc(100vw / 750); */

            font-size: calc(100vw / 7.5);
            /* 1rem = 100px */
        }

        .a {
            height: 0.9rem;
            background-color: #f00;
        }
    </style>
</head>

<body>

    <!-- 
        浏览器宽度   100vw   
        设计图 是  750px

        1px =  100vw/750
     -->

    <div class="a">呵呵</div>

</body>

</html>